Remove ISSUES_URL and NullCommand from brew cask help

Fixes #4726

* Tested to work on 10.10 and 10.9
* Changed the subtraction for symbols instead of strings

![screen shot 2014-06-06 17 37 53 0000](https://cloud.githubusercontent.com/assets/1889575/3203802/4e4d7278-eda1-11e3-837a-a97d485faa0c.png)